On Planets

I seem to have put my foot in it on someone's Facebook post (I'd post a link, but Facebook doesn't really like outside linking).

All I did was did was make an innocent remark about the number of planets in the solar system, and suddenly everyone is rallying to Pluto's defence. And fair play to them. If you want Pluto in your list of planets, as far as I'm concerned, you're welcome to it.

But for as long as I've been into astronomy (or at least into it enough to know what's going on in the solar system), I've had a problem with Pluto. But then, I've had a bit of a problem with the term "planet". Let me cover that first.

The word planet comes from the Greek verb πλανηθεί (planasthai), meaning to wander, referring to the way they moved across the sky. Anything which moved across the sky could be considered a wanderer, so from that point of view Pluto does deserve planet status, though you need a fairly powerful telescope to observe its wandering.

The trouble with this definition is it only takes account of what early astronomers could observe, the movement across the sky. Some moved faster or slower, but all followed a defined path, and if you were clever enough you could predict what that path would be.

Yet far from just being points of light that move around the sky, we now know quite a lot of detail about the composition of these objects, and they fall into a couple of distinct categories. It doesn't take a genius to see that the first four, Mercury, Venus, Earth and Mars are very different from the next four, Jupiter, Saturn, Uranus and Neptune, and it makes a lot of sense to separate them into "rocky planets" and "gas giants". There are also asteroids, such as Ceres, and not just the ones between Mars and Jupiter, some of which were regarded as planets at one point until they got demoted (I'm not certain whether this caused any outcry at the time).

But Pluto doesn't fit into either category. It's orbit is a wide ellipsoid, when the others are broadly circular, and it's made mainly of ice, with a small iron core. To understand it properly, you need to look at the solar system. We tend to think of it as a disk with the planets spinning around it. But That picture leaves out much of the solar system.

We could imagine the whole solar system as being about the size of a compact disc or DVD, and instead of imagining the planets covering the whole surface, imagine them fitting inside the clear plastic part near the middle. In fact, the inner solar system of the sun and rocky planets would fit in the hole at the centre, with the asteroid belt running around the edge of the hole. Pluto's orbit would lie around the silvery part before the bit where the data starts, though because it's not properly circular, it would encroach into the clear area of the CD.

But that still leaves all of the metallic area of the disc. We need to mentally divide this intp two regions with a circle running through the middle of the shiny bit. The inner part (from the clear part to our imaginary line) is known as the Kuiper belt, and contains many bodies like Pluto. It's currently estimated that there are about 70.000 objects in this region with a diameter of 100km or more. Like Pluto, they are mostly a metallic core with a mantle of ice. That could mean hundreds or even thousands of Pluto-sized bodies. If we were to classify these as planets, we could end up with a Rhyme I don't fancy learning!

The International Astronomical Union considered this to be a problem, so they held a conference to decide a better definition of a planet, and here's what they came up with (I've copied and pasted because I'm lazy):

1. Is in orbit around the Sun.2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.3. Has cleared the neighborhood around its orbit.

That's very clever. On the surface, it seems to bind together the rocky planets and gas giants, but relegates asteroids and Kuiper belt objects to Dwarf planet status.

This would seem to solve the problem, bit does have one or two problems, the most obvious being the meaning of clearing the neighbourhood. There are many astroids wandering through the solar system. For example, one group is known as the Apolloids, which are asteroids that cross Earth's orbit (and which will perhaps one day crash into Earth causing death and destruction). If Earth had cleared its orbit, these surely wouldn't be there.

So while I find the new definition interesting, it certainly isn't perfect, and it leaves itself open to interpretation. Just how many random objects are allowed to share its space?

But since planet is just a rather loose grouping of several different of object, there would be nothing wrong with saying "these nine objects are planets because we say they are,"

So while I'm happy to remove Pluto from my personal list of planets, if you want to keep on yours, that's okay by me.

Planets only have to have cleared their orbit of bodies of comparable size. This rules out Ceres (there are other asteroids that aren't much smaller) and Pluto (umm, Neptune), but the asteroids that cross the orbit of Earth (and moreso Mars) aren't a problem.

Don't get me wrong, I think it's a very clever rule, and I don't think Pluto should count as a planet (unless you want it to). But it's still a hack to try to tie together objects that don't have very much in common.